It is well known that alpha-olefins can be polymerized using a number of different catalyst systems. For gas phase polymerization, the catalyst is often supported on a solid porous carrier such as talc, resinous materials, or inorganic oxides. Generally, the purpose of supporting the catalyst is to improve the handling characteristics of the polymer product and to give better control of reaction rates.
Many supported catalysts are abrasive particularly under the high pressure conditions of many catalyst delivery systems. These abrasive catalysts tend to rapidly erode the valves of conventional delivery systems resulting in valve leaks. Leaky valves cause loss of precise process control and must eventually be replaced which is costly in terms of down time and replacement parts. Thus there is a need for valves that can better withstand the abrasive character of supported catalysts.
Although supported catalysts are abrasive, they generally have acceptable flow characteristics. In many instances, though, it is desirable to prepolymerize the supported catalyst. Prepolymerization is generally accomplished by treating the supported catalyst with a small amount of monomer and/or comonomer under polymerization conditions to form a coating of polymer on the supported catalyst particles which increases particle size and weight.
Prepolymerizing the supported catalyst improves both the catalyst particle strength and ultimately the polymer product characteristics, however, there is a concomitant degradation in catalyst flow characteristics due to the increase in particle size and particle surface irregularities. Consequently, when conventional feed systems are used to deliver prepolymerized supported catalyst, the system lines tend to clog. Thus there is a need for a reactor feed system that utilizes abrasion resistant valves and also resists clogging upon delivery of prepolymerized supported catalysts.
A number of catalyst feed systems for gas phase reactors are known to those skilled in the art. Well known systems include systems comprising a catalyst storage vessel connected to a feed chamber with filling and emptying valves in turn connected to a gas phase reactor. Typically the catalyst is conveyed from the storage vessel through the catalyst feed system to the reactor by maintaining the reactor at a pressure lower than that in the system. Valves incorporated in such systems allow a given quantity of catalyst to move from the storage vessel to a feed chamber or metering device and then to the reactor.
U.S. Pat. No. 4,162,894 describes a pressure equalized feed system incorporating a ball check feed valve and downstream positive shut-off valve for controlling intermittent feed of catalyst. U.S. Pat. No. 4,687,381 describes a feed system using a shut-off valve and metering device for periodic delivery of powdered catalyst. These and other conventional systems, however, do not overcome the problems caused by catalyst abrasion and poor catalyst flow. Thus, conventional feed systems are generally unsatisfactory when abrasive materials such as supported catalysts are used and/or when prepolymerized catalysts having poor flow characteristics are used. Under conditions of sustained intermittent delivery of supported catalyst to the reactor, conventional feed system valves rapidly wear down and begin to leak due to catalyst abrasion. Such leakage interferes with delivery control which consequently affects system performance, stability and ultimately the polymer product. In addition, prepolymerized supported catalysts tend to clog feed lines which also decreases delivery control and efficiency. Frequent repair and/or replacement of system valves and poor performance due to clogged lines is time consuming and expensive.
A desirable feed system would incorporate valves that are resistant to substantial leakage caused by abrasion and would resist substantial flow obstruction.